Air conditioning is widely used, but is subject to a number of disabilities in general.
Most air conditioning units which are in use employ vapour compression equipment with chlorofluorocarbon refrigerants, and these refrigerants sometimes leak into the atmosphere, and have a deleterious effect on the ozone layer. This matter is regarded as extremely serious and steps are being taken to limit use of CFC refrigerants.
When vapour compression systems are used, a great deal of energy is required to condition a space with sufficient control to achieve a condition of temperature and humidity within a so called "comfort zone" when the ambience is likely to change within wide ranges of temperature and humidity. In order to reduce energy consumption, it is common practice to recycle some of the air from the conditioned space, and in some instances as much as 85% of that air is recycled.
One of the difficulties which has been identified with recycled conditioned air is contamination. Not only do common germs spread throughout a building, but also dangerous chemical contamination such as formaldehyde and radon.
One of the objects of this invention therefore is to provide an air conditioning system that utilizes exhaust air, which is air that is moved from a conditioned space, to condition the supply air that is incoming to the conditioned space. This conditioning of the exhaust air, and heat transfer to the supply air is hereinafter referred to as "indirect air conditioning".
Another object of the invention is to provide a system which utilizes dehumidification of the exhaust air prior to the indirect conditioning of the supply air.
It is a still further object of this invention to provide a system which can be powered with the use of very much less electrical energy than is presently used in standard vapour compression refrigeration systems.
These objects are achieved by utilising indirect cooling or heating which employs heat exchangers that condition the exhaust air from a conditioned space, such that there is heat exchange between the exhaust air and the supply air. This exhaust air is eventually discharged to atmosphere, although in some instances recycling of some of the exhaust air may be possible.
Referring to the prior art, the closest reference to the present invention is disclosed in Australian Pat. No. 425702 in the name of the Commonwealth Scientific and Industrial Research Organization (inventor Donald Pescod), wherein a plate type heat exchanger having primary and secondary air flow passages therethrough is described. In particular a process of indirect cooling wherein water is introduced into the exhaust air within the plate type heat exchanger is described. The cooling of the exhaust air occurs only within the plate type heat exchanger wherein water is introduced within the secondary air flow passages through which the exhaust air travels. The cooling effect achieved is as a result of evaporation of the water by the exhaust air, thereby reducing the air temperature by transferring latent heat in the vaporization of the water.
However, the heat exchange in the abovementioned Australian Patent is only useful in dry and arid climates. Such a system has less applicability in more humid climates as the evaporative cooling process is less effective. Obviously, it would be necessary for dehumidification of the exhaust airstream for the heat exchanger in the abovementioned Australian Patent Application to be effective. Furthermore, this specification does not contemplate heating the air other than by simple heat recovery, and for an air conditioner to be useful, it must be provided with an effective heating mode.
The likely efficiency of an indirect air conditioning system is related to the heat transfer coefficient of the various heat exchangers, and with heat exchangers of the type suggested in the above-mentioned Australian specification, the heat transfer coefficient is likely to be in excess of 80%. In addition, research has found that in many applications an exhaust air quantity of about 80% of the air which enters the conditioned space is needed for that performance to be achieved in cooling, the remainder being air spillage to atmosphere. If, therefore, the air which is processed is exhaust air from the conditioned space, then there is an overall efficiency gain when compared with the presently used traditional vapour compression type systems. There are also further gains in efficiency in this invention in that in the heating mode the outdoor temperature at which a defrost cycle is needed is typically 7.degree. C. below that needed in a traditional reverse cycle air conditioner. A defrost cycle can in many instance comprise a traditional reverse cycle wherein the cooling coil is actually heated, and this is obviously very wasteful of energy.
Vapour compression equipment and direct evaporation equipment as presently used both require power for fans and offer considerable resistance to the supply air flow, and in the latter case, require large duct work to reduce the losses. However, by utilising the proposed indirect air conditioning system of this invention, the duct work can be reduced in size and the fan requirements can also be reduced.
Reference is also made to U.S. Pat. No. 4771611 (invented by the Applicant herein) which is a "hybrid" of evaporative and vapour compression air conditioning, in which only the supply air was conditioned or processed. In this invention, only the exhaust air is operated upon by the system, as the exhaust air after being conditioned is used to in turn condition the supply air.
When an air conditioner operates in a heating mode, by utilising an indirect air conditioning system and processing the exhaust air use can be made of waste heat, preheated air, directly heated air, and either a condenser of a vapour compression type of plant (if used) or, advantageously, the heat available from dehumidification of the exhaust airstream. This therefore avoids mixing contaminated or stale exhaust air with the fresh air supply while exchanging heat between the two air flows. Likewise in a cooling mode, stale exhaust air which may well have been further contaminated in a desiccant type dehumidifier is unable to be mixed with a fresh supply air which is in turn introduced into a conditioned space.